1. Technical Field
The present invention relates generally to an improved communications system and in particular to a method and apparatus for configuring a communications system. Still more particularly, the present invention relates to a method and apparatus for designing soft handoff regions in a wireless communications system.
2. Description of the Related Art
Many of the present digital telecommunications networks of the world are based on time-division multiple access (TDMA) and use transmission and switching products like digital switches and pulse coded modulation (PCM) transmission at E1 or T1 rates. TDMA users share the radio spectrum in the time domain. An individual user is allocated a time slot and, during this time slot, the user accesses the whole frequency band allocated to the system (wideband TDMA) or only a part of the band (narrow band TDMA). In TDMA, transmission take place in bursts from a mobile station to a base station in an uplink path with only one user transmitting to the base station at any given time. In the downlink path from the base station to the mobile station, the base station usually transmits continuously with the mobile station listening only during the assigned time slot. TDMA channel multiplexes the bids from a number of users. This type of system requires transmission at a higher bit rate over a radio frequency channel.
Another technology that is becoming more widely used is code-division multiple access (CDMA) in a spread spectrum. Spread spectrum techniques spread the bandwidth of the transmitting signal over a spectrum or band of frequency much larger than the minimal band with required to transmit the signal. CDMA has an ability to lock out conflicting signals, which may allow it to share a system with other radio signals without interference.
Wireless technology is an important component of the global information infrastructure. To exploit the full potential of this technology, regulators and administrators of all countries are re-evaluating their spectrum allocation policy. A large segment of the spectrum around two GHz bandwidths has already been released for the use of wireless networks. More spectrum is being allocated by the International Telecommunications Union (ITU) and countries like the United States of America and Canada around the 7-10 Ghz range to use wireless technology for broadband wireless applications. Wireless networks are being built all over the world to handle a large volume of traffic and mobility.
When a mobile station moves between cells, a handoff occurs, a process that allows a call in progress to continue as the mobile station moves between cells. Handoffs may be based on received signal strength or signal-to-interference ratio (measured either at the terminal, the mobile station, or both) or may be based on network resources management needs, such as, for example, a forced handoff to free resources to allow an emergency call to be placed. Handoffs can be classified as soft or hard. A soft handoff occurs when the mobile station call is passed to a target base station without interrupting communications with the current base station serving the call. In a soft handoff, the mobile station communicating with two or more base stations simultaneously with the signals from the base stations to the mobile station being treated as multiple path signals that is what coherently combined at the mobile station. A hard handoff occurs when the communication to the mobile station is passed between disjointed radio systems, different frequency assignments, or different air interface characteristics or technologies.
Within wireless cellular communications systems, the architecture seeks to provide an efficient use of available channels by using low-power transmitters to allow frequency reuse. Wireless cellular communications systems are designed to operate with groups of low-power radios spread out over the geographical service area. Each group of radios serves mobile stations presently located near them. The area served by each group of radios is called a "cell". The power transmitted is chosen to be large enough to communicate with mobile stations located near the edges of the cell. The radius of the cell may be chosen to be, for example, 26 kilometers in a start up system with relatively few subscribers and down to less than 2 kilometers for a mature system requiring considerable frequency reuse. As the traffic grows, new cells and channels are added to the system. In the designing of cells, however, no design criteria or methodology are present determining soft handoff regions for a particular cell.
Therefore, it would be advantageous to have an improved method and apparatus for designing and implementing soft handoff regions in cells in a communications system.